Fair Value Measurements (Tables)

6 Months Ended

Jun. 30, 2015

Fair Value Disclosures [Abstract]

Schedule of assets and liabilities that are recognized at fair value on a recurring basis

The following table presents assets and liabilities included in our consolidated balance sheets that are recognized at fair value on a recurring basis, and indicates the fair value hierarchy utilized to determine such fair value as of June 30, 2015 and December 31, 2014.

	Balances as of June 30, 2015
	Total			Quoted Market Prices in Active Markets (Level 1)		Significant Other Observable Inputs (Level 2)			Significant Unobservable Inputs (Level 3)
	(in millions)
Cash and cash equivalents	$	72.0		$	72.0	$	—		$	—
Unrealized gains on natural gas derivatives	3.1			—		3.1			—
Total assets at fair value	$	75.1		$	72.0	$	3.1		$	—
Unrealized losses on natural gas derivatives	$	(1.1	)	$	—	$	(1.1	)	$	—
Total liabilities at fair value	$	(1.1	)	$	—	$	(1.1	)	$	—

	Balances as of December 31, 2014
	Total			Quoted Market Prices in Active Markets (Level 1)		Significant Other Observable Inputs (Level 2)			Significant Unobservable Inputs (Level 3)
	(in millions)
Cash and cash equivalents	$	113.0		$	113.0	$	—		$	—
Unrealized gains on natural gas derivatives	0.1			—		0.1			—
Total assets at fair value	$	113.1		$	113.0	$	0.1		$	—
Unrealized losses on natural gas derivatives	$	(3.6	)	$	—	$	(3.6	)	$	—
Total liabilities at fair value	$	(3.6	)	$	—	$	(3.6	)	$	—